I think he's referring to the Nikon TC17 ED (?) which is an add on auxillary lens that screws into the filter threads. It's actually an excellent TC, with ED glass, but is pretty heavy and expensive (@ $360). They are made to extend the reach of fixed lens cameras.

Unlike TCs that fit between the lens and the camera, these TCs do not change the effective aperture of the main lens (they have a significantly larger objective diameter, and gather more light). They can, however, cause problems with lenses that don't have internal focusing, as they add a significant amount of weight that now needs to be moved by the focusing motor.

The Nikon is probably the best one made, but IIRC, all of them will cause some increased CA and PF, and they also will reduce sharpness and contrast to some degree.

Hi Everyone. I have recently bought myself the Sigma 50-500mm EX DG (I have an understanding wife) and can highly recommend it. I looked at the 135-400 but was put off by some of the reviews on it.

Had the Sigma 70-300 (non-Apo version) for a while and was pleasantly surprised with its sharpness and colour rendition when stopped down smaller than F8, so would expect the Apo DG to be a good investment. Also tried the Tamron 28-300 LD for a couple of months but it suffered badly from CA and was never really as sharp as the Sigmas.

The 50-500 is a heavy beast (nearly 2KG) and not overly fast either. But it is very, very sharp right across the range which is quite amazing for such a wide zoom range.

The attached photo was shot at 500mm wide open (F6.7 on my DS). This lens is definitely a keeper.

That's a great lookin shot Greg! I think for starting out that lens my be kinda big for me. Glad to hear another endorsment for the 70-300 tho. I think I'll get that for my walk around lens and maybe something like the Sigma 17-70 f2.8 for low light and indoors. And possibly a LONG prime.....any suggestions?